Lesson 4: Properties and Functions of the Living Cells

Cells resemble one another anatomically and they have certain requirements which are satisfied in much the same way.

Unit 1: Nutrition
All the activities of an organism that have to do with securing, consuming digesting and absorbing food may be considered as nutrition. At the higher organism level, this involves a complex of organ systems which we are not concerned with now. We are only concerned with the ways in which a cell may receive nourishment.

There are two methods by which cells acquire food.

(i) Those cells that are able to synthesize their own food, that is, manufacture all the complex molecules that are used for building materials and for energy, are said to possess autotrophic nutrition and are called autotrophs. Most autotrophs are photosynthetic, although a few are chemosynthetic.

The autotrophs include green plant cells and purple bacteria which obtain their energy for building food molecules from the sun (photosynthetic). A few bacteria are able to gain energy for food synthesis by oxidizing certain inorganic compounds (chemosynthetic).

(ii) Those cells that are incapable of manufacturing their own complex organic compound from simple inorganic nutrients and must depend on outside sources for their food are said to have heterotrophic nutrition. They are said to be heterotrophs.

Regardless of how the cell comes by its raw food molecules, once they are inside the cell, the process by which energy is released from these molecules and the way products are synthesized are remarkably similar in all cells.

Unit 2: Cellular Respiration
Cells break down carbohydrates, proteins, fats and oils to release energy by a process called respiration. The complete respiratory degradation of a compound such as glucose to carbon dioxide and water involves a very large number of reactions and there are two stages in the process.

(i) Anaerobic Respiration, which does not utilize oxygen and can take place whether or not it is present
(ii) Aerobic respiration, which requires oxygen ‘during the breakdown process.

Energy release: Higher percentage of energy initially present in food substance is released through aerobic respiration than in anaerobic respiration. Oxygen is therefore essential to the life of most cells or organisms. With each cell, there are molecules which are used as store houses for energy. Each of these molecules is called Adenosine Triphosphate or ATP.

A cell constantly used up energy-rich ATP converting ¡n into relatively energy-poor molecule Adenosine Diphosphate or ADP. Further cell respiration supplies more useful energy to ADP thus converting ¡t into ATP

Glycolysis.
The energy required to form ATP is obtained from the oxidation of glucose. This energy is released from the glucose molecule through a series of processes. It is necessary to degrade the glucose molecule in a slow orderly fashion so that the energy can be stored in ATP at body temperature.

If it was oxidized rapidly the energy would be lost in form of heat and thus could not be made available to the cell for doing all the necessary jobs that it must do.

Glucose is first converted to fructose-6-phosphate and then to frustose-1,6-disphosphate. The energy for these reactions are obtained from ATP. The fructose-1,6-disphosphate then passes through a series of steps during which it is degraded finally reaching the three-carbon compound pyruvic acid.

During these steps, 2 molecules of ATP are used and 4 molecules of ATP are produced; hence there is a gain of 2 molecules of ATP. The series of steps taken so far in degrading the glucose molecule has yielded very little energy and no oxygen has been used. For that reason, we say this is an anaerobic process, and the over-all reaction to this point is called glycolysis.

The pyruvic acid produced by this anaerobic breakdown can be further degraded all the way to carbondioxide and water if oxygen is present by reaction called the Krebs citric acid cycle. But in the absence of sufficient O2, the pyruvic acid may form CO2 and ethyl alcohol in some kinds of organisms and latic acid in others.

Unit 3: Excretion

Excretion and Excretory Products
Excretion is the process by which waste products of metabolism or excess amount of materials in the cell are removed. This is very important because often times most waste products are toxic and poisonous to the cells especially the nitrogenous wastes.

One of the reasons most wastes do not present serious problem of removal from the cell is because most of them are soluble in water and such can be easily excreted from the cell surface by simple diffusion.

The major waste in the cell includes carbondioxide and water which are produced during cellular respiration.

1. Oxygen is produced during photosynthesis; even though it is required for respiration, in this circumstance it is a waste which must be expelled from the cell.
2. Nitrogenous wastes are produced during the breakdown of excess amino acids or breakdown of certain proteineous cellular parts which have become old or worn out.
3. Excess water, salts and sometimes highly toxic substances which are of no use, but have found their way into the cell by diffusion or in conjunction with needed nutrients.

Excretory Organelles of the Cell
The main excretory organelle of the cells is the vacuole. In cells like those of Amoeba the vacuole is a membrane bound sac into which waste mainly water that penetrate the cell by osmosis is constantly discharge or expelled when its accumulation has gotten to a critical level.

In Paramecium and Euglena the contractile vacuole has a part from the main chamber smaller channels. But generally, many cells would excrete waste mainly by diffusion they are being produced without accumulation in a contractile vacuole, especially the cells of higher animals.

Unit 4: Growth
Growth is one of the major characteristics of living things i.e. the cell. To an ordinary person, growth is just an increase in size, but to the cell and organism it is more than.

Growth once happened remain i.e. growth is permanent. Moreover growth cannot be reversed. Based on the two parameters above, growth can be defined as a ‘permanent and irreversible’ increase in size and length of the cell or organism resulting from increase in weight and number of cells of an organism.

(i) Basis of Growth

For growth to take place, three major things are involved these are:

1. Cell division (mitosis).
2. Cell enlargement
3. Cell differentiation

Cell Division
Cell division can be explained in terms of multiplication of cells. In order to multiply, cells undergo cell division. One cell divides into two, two divides to four, four divides to eight, eight to sixteen and so on.

Where the division is mainly in body cells (somatic), daughter cells have the same number of chromosomes. Cell division is the first process by which growth occurs.

Cell Enlargement
This is the second process by which growth occurs. If there is increase in the number of cells i.e. from one to two and two to four, there must be a means to accommodate the newly formed cells with the existing ones to avoid their being choked up. The process of the cell enlargement which brings about a new size is otherwise referred to as growth.

Cell Differentiation
This is the last phase of growth process where cell divides it cells for new size (enlargement). The new size needs to be classified and sorted out to differentiate between old number of cell and the new cell number. This way, the mark difference could be discussed as fresh weight which is termed growth.

Regulation of Growth by Hormones
Certain factors and parameter can influence control growth. One of it is the hormone. Growth in plant and animals are coordinated by hormones to create a balance in growth i.e. proportionality in the parts of organisms,

As one part grows, the other part also should maintain a balance, for example in plant: as the shoot grows, the root must also grow. Plant for instance has a specific growing point which is called meristematic regions (area of growth).

This is the place where cells can divide and it is usually at the tip of shot and root (Apical growth). This growth area in plant is where the plant hormone is located. The plant hormone is called auxins.

In animals, growth is not continuous throughout life; it occurs uniformly all over the animal body. Hormones aid animal growth in large measure and hormones are located in animals’ body to effect the growth.

The effect is such that under secretion of these hormones leads to poor growth (Dwarfism) while over secretions leads to excessive growth (Giantism).

Hormones that Regulate Growth in Plants.
The following hormones (auxine) influence different aspects of growth in plants.
a. Inclole (iii) acetic acid (IAA)
b. Cytokinins
c. Gibberellins
d. Abscissic acid

Hormones that Regulate Growth in Animals
The following hormones influence different aspects of growth in animals
a. Pituitrin
b. Thyroxin

Factors Affecting Growth
(i) Heredity
(ii) Growth hormone
(iii) Food nutrients
(iv) Water
(v) Vitamins
(vi) Temperature
(vii) Light
(viii) Differentiation

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